#include <cassert>
#include <cmath>
#include <algorithm>
#include <iostream>
#include <iomanip>
#include <climits>
#include <map>
#include <queue>
#include <set>
#include <string.h>
#include <vector>

using namespace std;
typedef long long ll;

struct Food {
  int p;
  int q;

  Food(int p, int q) {
    this->p = p;
    this->q = q;
  }
};

int N, D;
vector<Food> foods;

bool f(int x) {
  int val = 0;
  bool checked[N];
  memset(checked, false, sizeof(checked));

  for (int d = 0; d < D; ++d) {
    int best_i = -1;
    int max_v = INT_MIN;

    for (int i = 0; i < N; ++i) {
      if (checked[i]) continue;
      Food &food = foods[i];
      if (val - food.p < x) continue;
      int v = food.q - food.p;

      if (max_v < v) {
        max_v = v;
        best_i = i;
      }
    }

    if (best_i == -1) return false;
    checked[best_i] = true;
    val += max_v;
  }

  return true;
}

int main() {
  cin >> N >> D;

  for (int i = 0; i < N; ++i) {
    int p, q;
    cin >> p >> q;
    foods.push_back(Food(p, q));
  }

  int ok = -100000000;
  int ng = 1000000;

  while (abs(ok - ng) >= 2) {
    int x = (ok + ng) / 2;

    if (f(x)) {
      ok = x;
    } else {
      ng = x;
    }
  }

  cout << ok << endl;

  return 0;
}